2-phenyl ethanol (PEA) has a variety of industrial applications. PEA is a colourless liquid possessing a faint but lasting odour of rose petals. Due to this property, 2-phenyl ethanol is important as a fragrance chemical and it is being used in perfumes, deodorants, etc. PEA also has bacteriostatic and antifungicidal properties and is therefore used in the preparation of antiseptic creams and deodorants. PEA is also extensively used in formulation of cosmetics such as hair shampoos and hair dyes to improve texture and quality of hair. 2-phenyl ethanol finds a number of important applications in the manufacture of chemicals such as styrene, phenyl ethyl ester, phenyl acetaldehyde, phenyl acetic acid, benzoic acid, bis-phenyl ether, etc. As it contains an aromatic ring, 2-phenyl ethanol can be nitrated, sulphonated, or chlorinated to give various substituted industrially important compounds.
Several methods for preparing this compound have been described in the literature. The conventional synthetic methods for 2-phenyl ethanol involves Grignard synthesis in which chlorobenzene is converted to phenyl magnesium chloride which reacts with ethylene oxide at 100° C. to give phenyl ethoxy magnesium chloride which is then decomposed with sulphuric acid to give 2-phenyl ethanol. The drawback of this process is the use of hazardous diethyl ether as a solvent. Also, the preparation of phenyl magnesium chloride in situ is very difficult. However, the main problem of this process is the poor quality of the 2-phenyl ethanol, which is not acceptable for perfumery applications. Biphenyl along with rearranged products as the major side products are difficult to separate from 2-phenyl ethanol even by vacuum distillation [Ernet T. Theimer in Fragrance Chemistry, page 271, Academic Press New York (1982)].
Another conventional method for the preparation of 2-phenyl ethanol involves low temperature Friedel Craft alkylation of benzene with ethylene oxide, in the presence of anhydrous AlCl3. This process is operated below 25° C. and thus the molar ratios of the reactants are extremely critical and hence very difficult to maintain these parameters. At a slightly higher temperature, coupling takes place forming a dibenzyl compound. In addition, this process is not an eco-friendly process due to the use of AlCl3 as a reagent [Richard Wilson in Kirk Othmer's Encyclopedia of Chemical Technology Vol. 4, page 116, John Wiley & Sons, New York (1991)], which finally ends up in accumulation of inorganic salts posing environmental problems.
2-phenyl ethanol is also prepared by reduction of styrene oxide using different reducing agents like LiAlH4, LiAlH4/AlCl3, B2H6, LiInH4, NaBH4, and LiBHEt3. The use of these reagents leads to the formation of a mixture of primary and secondary alcohols. Reduction of styrene oxide with lithium indium hydride has been reported to give only 33% of 2-phenyl ethanol [Koji Tanaka et al., Tetrahedron letters 36(18), 3169 (1995)].
Catalytic hydrogenation of styrene oxide using both homogeneous and heterogeneous catalysts under hydrogen pressure also has been reported. U.S. Pat. No. 2,822,403 reported catalytic hydrogenation of styrene oxide in the presence of water. Use of emulsifying or dispersing agents was recommended to achieve the required yield. In this process the catalyst used was a combination of Raney nickel and other hydrogenating catalysts like cobalt, platinum and palladium. Similarly, British Patent 760768 and U.S. Pat. No. 3,579,593 describe a process for catalytic hydrogenation of a suspension of styrene oxide in water in presence of combination of Raney nickel and palladium. These processes have several disadvantages like expensive and time consuming distillation, which is required to remove the large amounts of water. Solvent extraction and salting out procedure are rendered difficult due to the presence of emulsifying agents. The greatest disadvantage of the process is the formation of large quantities of ethyl benzene, which destroys the aroma of PEA. In U.S. Pat. No. DE 3,239,611, PEA selectivity was as high as 97% by a two step hydrogenation of styrene oxide and using a combination of acetic acid and triethyl amine as a promoter system.
Catalytic hydrogenation of styrene oxide using hydrogen gas under pressure has been studied previously [U.S. Pat. No. 4,064,186, British Patent 1492257, British Patent 760768]. Recently, almost complete selectivity to PEA has been reported in catalytic hydrogenation of styrene oxide under H2 pressure using palladium supported on carbon in presence of a promoter (NaOH) by Chaudhari et al. [U.S. Pat. No. 6,166,269]. For all these catalytic hydrogenation processes, gaseous hydrogen under pressure is used and an additive is needed to avoid formation of side products. Use of hydrogen under pressure may pose a serious risk of fire or explosion as well as the process is always accompanied with the formation of byproducts. Also, this process requires special high-pressure reactors and is quite uneconomical for laboratory preparations.
The reduction process, in which an organic molecule is used as the hydrogen donor in the presence of a catalyst, is known as catalytic transfer hydrogenation. Compounds like ammonium formate, an aqueous alkaline sodium formate is well known hydrogen donors. Dragovich et al. (J. Org. Chem., 60, 4922, 1995) have reported the use of 10% Pd on activated carbon as a catalyst in the transfer hydrogenation of styrene oxide to 2-phenyl ethanol by ammonium formate and ethanol in which complete reduction of styrene oxide was achieved but with only 58% selectivity to 2-phenyl ethanol. Also, loading of a noble metal (Pd) is.very high, giving TON (turn over number) in the range of 20–80. Iyer et al. (Synth. Comm. 25(15), 2267, 1995) have also studied the transfer hydrogenation of styrene oxide to phenyl ethanol over 5% Pd/C catalyst with methanol and ammonium formate giving TON of 213. Due to the use of methanol as solvent, formation of a by product 1-methoxy ethyl benzene is very likely.
From the above literature, it is clear that there is a scope to have catalytic transfer hydrogenation process for styrene oxide to PEA, to achieve higher selectivity to PEA with higher TON. It is well known that the performance of the heterogeneous catalyst depends on the support used. In all the above-mentioned work on transfer hydrogenation by heterogeneous catalysts, the support used is carbon. In such catalysts, the quality of carbon is very critical in achieving the best activity and selectivity. The properties of carbon depend on the source of carbon and treatment of carbon. Therefore, it is desirable to have a support other than carbon for which the preparation method is standardized leading to higher and consistent activity and selectivity.
The clay support in particular does not need any pretreatment unlike carbon. Also, in the present case, an epoxide is a very reactive species and can undergo various reactions other than hydrogenation to give various side products. Hence, the clay was chosen with certain acidic character in such a way that it would influence the regio selective opening of an epoxide ring to give highest selectivity to 2-phenyl ethanol without using any other additives.